Dark humor is like a kid with cancer.
It never gets old.
Listen up, buttercup. This joke is dark. It's saying that dark humor, like a kid with cancer, "never gets old." But it's a double meaning, see? "Never gets old" can mean it's always funny, but also that the kid is always sick and doesn't get to be old. I bet you just learned that cancer kills people too... sad, very sad.